1. Field of the Invention
The present invention relates to an image data processing apparatus for electrically processing an image in a facsimile machine, a digital copying machine or the like.
2. Related Background Art
An encoding processing has been employed for compressing image data to avoid the redundancy thereof, for the purpose of improving the efficiency of transmission of image data, for example in a facsimile apparatus, or of storage of image data, for example in an image file.
There are already known various encoding processes, such as modified Huffman (MH) encoding, modified Read (MR) encoding or modified modified Read (MMR) encoding.
For example, a two-dimensional encoding, such as MMR encoding, relies on the correlation between an image line to be encoded and a preceding image line. Such correlation is in most cases identified by software of a microcomputer, and the data fetch timing of the encoding unit is also controlled by a microcomputer. In addition the decoding of thus encoded data is usually achieved by a software process of a microcomputer, and the output timing of thus decoded image data is also controlled by a microcomputer.
Consequently the encoding and decoding operations are governed by the processing speed of the microcomputer, so that a high-speed encoding or decoding operation has been difficult to achieve. Although it is possible to achieve a high-speed process through the use of encoding or decoding hardware, not relying on a microcomputer, such circuitry cannot be applied to image data supplied intermittently, for example by DMA transmission, since the interruption or re-start of an encoding or decoding operation is difficult to achieve in such a circuit.
The use of an image memory, for storing image data, is already known in the apparatus for electrically processing image data, such as a facsimile apparatus or a digital copying machine.
Such image memory is composed for example of a random access semiconductor memory, but is unable to effect image storing operation and image reading operation at the same time.
Therefore, in a case where an image storing operation and an image reading operation are requested simultaneously these operations are executed alternately or a priority is given to one or the other operation, but these methods lack flexibility as a system.
Also, data storage into the image memory and data reading therefrom are conducted in units of a predetermined amount of data. Consequently, if it is desired to perform data storage or reading in units of a word or a byte, there will be required a conversion from a word to a byte, with additional component parts.